Distillation remains the most widely used method for separating a mixture into its components or separating a few valuable products from a mixture. Distillation involves stripping and rectifying, both typically conducted in the same tower at different heights along the tower. In the lower portion of the tower, below the feed tray, a mixture is heated causing it to boil. The generated vapors pass upwardly through the column. At the feed tray, a feed mixture in liquid form is injected into the column. This liquid flows downwardly, contacting the rising vapors. The vapors preferentially strip the more volatile components of the mixture from the downwardly flowing liquid. Vapors continue to rise upwardly into the upper rectifying portion of the tower and pass completely out of the column. A portion of these vapors are condensed and re-injected at the top into the column near its highest point. This condensed liquid contacts the upwardly flowing vapors causing the less volatile material to condense. The separation occurring in the stripping section is affected by the vaporization of the more volatile components. The separation in the rectifying section is caused by the condensation of less volatile material.
Often it is desired to separate several components from a multi-component mixture feed. To accomplish this, several columns can be aligned in series to remove one pure component or one boiling range from each column. When a feed mixture is to be separated into three components, an alternative to a series of continuously fed distillation units is fractionating one pure cut either in the kettle or overhead and taking a third cut from a side takeoff location. However, with such a multi-component mixture, a column with a side takeoff generally produces only one pure product. Therefore, when it is desired to have two or more high purity products from a fractional distillation system, typically, a series of columns has been used. It would be highly desirable to have a single distillation column capable of producing two or more high purity products.